This invention relates to digital communications systems and in particular to systems embodying asynchronous transfer mode (ATM) technology. The invention further relates to the application of PNNI (Private Network Network Interface) signalling to control an ATM adaptation layer switching network.
The asynchronous transfer mode (ATM) technology is a flexible form of transmission which allows any type of service traffic, voice, video or data, to be multiplexed together on to a common means of transmission. In order for this to be realised, the service traffic must first be adapted typically into 53 byte cells comprising 5 byte headers and 48 byte payloads such that the original traffic can be reconstituted at the far end of an ATM network. This form of adaptation is performed in the ATM adaptation layer (AAL).
A recent development has been the introduction of the AAL-2 adaptation layer. This adaptation layer has been optimised to accommodate the demands of low bit-rate communications representing the increasing trend to greater voice compression. The adaptation layer is a multiplex of users in a single ATM connection where each user""s information is carried in short packets or minicells each with a header identifying the user channel and incorporating ancillary control information. This constitutes a dynamic trunk group of users in a single connection.
As telecommunications networks increase in complexity and carry increasing volumes of traffic, the current procedures for setting up connections between subscribers are limiting the performance of these networks. In particular, congestion may be caused by attempting to connect to a subscriber who is already busy, or by attempting to choose a route through an already congested part of the network. Thus equipment and resources can be wasted in attempts to set up calls which cannot be completed. A further problem is that of scalability. As the network expands to accommodate increased traffic and a larger number of subscribers, there is an increasing need to facilitate integration of new equipment into an existing network without simply increasing the congestion problem. Moreover, careful planning is required to ensure that calls can be routed through the network. This requires an ad-hoc distributed routing decision policy which limits the flexibility of the routing process as the call routing must be collocated with the switching node fabric.
The object of the invention is to minimise or to overcome these disadvantages.
A further object of the invention is to provide an improved method of operating a telecommunications network.
According to one aspect of the present invention there is provided a asynchronous transfer mode adaptation layer switching network having independent call routing and connection control for setting up connections across the system.
According to another aspect of the present invention there is provided a distributed telecommunications exchange system having means for determining at a call source the current status of the system whereby to effect routing of a multimedia call across the system.
According to another aspect of the invention there is provided a method of communicating resource availability to maintain performance of an asynchronous transfer mode adaptation layer switching network under overload conditions.
According to a further aspect of the invention there is provided a method of routing telecommunications traffic in a system including an asynchronous transfer mode (ATM) network having uncommitted bandwidth, and a plurality of adaptation layer switches (ALS) coupled to the ATM network, which adaptation layer switches comprise a group adapted to function as an adaptation layer switching network whose fabric and control are distributed over the group, the method including determining the current system status whereby to set up multimedia calls across the network based on that status determination.
We have found that adaptation layer switching can be incorporated into an ATM private network network interface (PNNI) reference architecture to provide a mini channel connectivity layer and a routing architecture for establishing connections in the adaptation layer.
The arrangement and method facilitates both scalability of the network and the separation of call routing and connection control. The advantage of this separation of the two functions can be exploited by making routing decisions at the outset. Dynamic trunking allows the routing decisions to be independent of the connection control. With fixed trunk networks, voice routes are made up of small trunk groups that pertain to physical links which may be diversely connected. The status of these connections will affect the final route available. Because the status is distributed, so must be the routing, and hence there can then be no separation.
The arrangement and method further facilitate the simplification of the PNNI model to reduce hierarchy, obviate crank back and alternate path routing, and simplify call admission policy on a service related basis.
PNNI may be applied to dynamic trunking in the adaptation layer using ATM connections to represent the physical bearer, but additionally the virtual connectivity allows dual homing, load-balancing, diverse routing and virtual connections may be re-established automatically over alternative physical paths to maintain connectivity. This enables the construction of an extremely robust network.
Reference is here directed to our co-pending application No. GB-9614138.7. This application describes a telecommunications system comprising an asynchronous transfer mode (ATM) network having uncommitted bandwidth, and a plurality of adaptive grooming routers (AGR) coupled to the network. The AGRs comprise a group adapted to function as a virtual transit exchange whose fabric and control are distributed over the group. The virtual transit exchange comprising the AGRs incorporates independent connection control and call routing functions and has means for determining the current system status whereby to set up narrow band connections across the ATM network based on that status determination.